Phase 1b/2 Study of IV Sarilumab in Adult With RA

This is a Phase 1/Phase 2 study with: * 5-arms design for Part A; * and a single arm for Part B. The purpose of this study is to measure PK parameters and safety with sarilumab intravenous (IV) with or without concomitant oral conventional synthetic Disease-Modifying Antirheumatic Drugs (csDMARDs) in male and female participants with moderately to severely active rheumatoid arthritis aged 18 years of age or older. Study details include: * The study duration will be up to 64 weeks. * The treatment duration will be up to 6 months for each study phase. * Part A has 10 visits, including a post-treatment end of study (EOS) follow-up visit. * For participants entering the open label extension to receive the approved 200 mg sarilumab every two weeks (Q2W) dose, there will be 3 additional study visits. * For the intra-study sarilumab 200 mg Q2W subcutaneous (SC) arm, participants will be evaluated over the course of 24 weeks plus post-treatment EOS follow-up visit following the schedule of activities (SoA) of Part A from Day -1 to Day 29 (total of 8 visits) and the SoA of Part B from Week 4 to Week 24 (total of 8 visits) and a post-treatment end of study (EOS) follow-up visit at Week 30 (Part B) for a total of 17 visits, including a post-treatment EOS follow-up visit. * Part B has 13 visits, including a post-treatment EOS follow-up visit.